Tires can vary a lot in actual size even if nominally the same size. A taller tire will compute to lower economy but actually be better on consumption. The only way to be sure is to measure the actual diamater of before and after.

Or you could check against the mile markers on the interstate.

Tom W

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[SIGPIC] Diesel loving autocrossing grandpa Architect. 08 Dodge 3/4 ton with Cummins & six speed; I have had about 35 benzes. I have a 39 Studebaker Coupe Express pickup in which I have had installed a 617 turbo and a five speed manual.[SIGPIC]

..I also have a 427 Cobra replica with an aluminum chassis.